NK cells defend against viral infections and cancer, but little is known about the bone marrow cells (BMC) that become NK cells. The over-all goal of this revised competitive renewal is to better define cells in early stages of NK cell development and the factors that control their function. Aim 1 will focus on further characterization of the NK cell precursor by a] producing new mAbs selective for NK cell precursors; b] using the new and known mAbs to further enrich and define NK cell precursors in earlier and later stages of development. Aim 2 will focus on the stromal interactions and factors which regulate function of NK cell precursors by a] evaluating the ability of specific stromal cell lines to support NK cell development and b] identifying stromal-precursor cell interactions and cytokines that regulate proliferation and/or differentiation of NK cell precursors. These studies will provide needed information to evaluate the effects of various clinical treatments or conditions on NK cell precursors, and to manipulate the proliferation and differentiation of these precursors for therapeutic gain.